Emit nullable_item last for OneOfBuilder

[frederikrothenberger]

This PR changes the order in which the #nullable_item is emitted when processing the OneOfBuilder. This leads to much better example values in the Swagger UI, as by default the example just picks the first one of the options.

This change is inspired by this comment: #1260 (comment)

What we could at least do is to change the order of the types in oneOf to make the null variant below, which will help the Swagger UI to use the correct spec as an example because it chooses the first one from the list.

Please take a look!